1. Field of the Invention
The present invention relates to a superconducting magnet apparatus and a method for magnetizing a superconductor and, more particularly, to an apparatus that causes a bulk high-temperature superconductor to capture a great magnetic field and makes it possible to use the superconductor as a magnet and a method for magnetizing the superconductor.
2. Description of the Related Art
Through structure control, some high-temperature superconductors formed from, for example, yttrium (Y)-system materials, have been developed that are able to capture great magnetic fields exceeding 1 T, which is impossible for permanent magnets to capture, at a liquid nitrogen temperature level. These superconductors are capable of capturing increased magnetic fields if they are cooled to lower temperatures. Moreover, since property improvements are expected due to developments in the field of materials, use of the superconductors as strong magnets is lately considered.
There are mainly two methods for magnetizing a bulk superconductor: a so-called FC (Field Cooling) method that cools a bulk superconductor to the superconduction transition temperature Tc of the superconductor or a lower temperature while applying a magnetic field to the superconductor; and a so-called ZFC (Zero Field Cooling) method that cools a bulk superconductor to its superconduction transition temperature or lower and then applies a magnetic field to it from the outside so that the magnetic field penetrates into the superconductor. In either method, it is necessary to apply a magnetic field at least equal to a magnetic field that the superconductor is desired to capture, to the superconductor at least once. Furthermore, it is necessary to maintain the temperature of the superconductor at a temperature equal to or lower than the temperature at the time of magnetization, in order to maintain the magnetic field captured by the superconductor.
The FC magnetization method has normally been employed to cause a high-temperature superconductor to capture a magnetic field for the purpose of, for example, evaluating the characteristics of the superconductor. For example, a technology disclosed in Japanese Patent Laid-Open No. Hei 7-111213 uses the FC method to cause a superconductor to capture a magnetic field, and produces a magnet by combining the superconductor and a coil.
In the ZFC magnetization method, on the other hand, after a superconductor is cooled, an external magnetic field is slowly applied to the superconductor and then slowly reduced to zero. Since the superconductor has already been cooled to the superconducting state at the time of application of the external magnetic field, a certain amount of the external magnetic field applied is expelled. Therefore, the ZFC method requires application of a greater magnetic field than the FC method. This is part of the reason why if a steady magnetic field is to be used for magnetization, the FC method, not he ZFC method, is normally employed for practical purposes.
Besides the foregoing methods, which simply turns a bulk superconductor directly into a magnet, another magnetization method is disclosed in Japanese Patent Laid-Open No. Hei 5-175034. In this method, a bulk superconductor is formed into the shape of a coil, and the coil-shaped superconductor is magnetized by supplying electricity to the superconductor.
The conventional FC method requires that a steady magnetic field be applied to a superconductor while the superconductor is being cooled. However, the steady magnetic field can be produced only in a small magnitude if a simply-constructed magnetic field generator is employed. Therefore, as long as a simple generator is employed in the FC method, it is normally impossible to cause a superconductor to capture a magnetic field that considerably exceeds the magnetic field of a normal permanent magnet.
A Nb--Ti superconducting coil can be used in the FC method to produce a great steady magnetic field to be applied to a superconductor. However, since the Nb--Ti superconducting coil needs to be cooled to a very low temperature, the entire apparatus for performing this method normally needs to be increased in size and complexity in order to cause the superconductor to capture a great magnetic field.
Furthermore, since the superconductor must be cooled while being subjected to a magnetic field, the FC method requires a long time for magnetization. In addition, after magnetization, the superconductor must be continually cooled even when installed for use, thus considerably limiting the location of use. Therefore, the FC method is not uitable for the purpose of using a superconductor as a strong magnet disposed inside an apparatus or thy like.
If the ZFC method uses a steady magnetic field, the method suffers from problems similar tn those of the FC method. Moreover, since the ZFC method requires a greater applied magnetic field than the FC method, the problems become more remarkable in the ZFC method.
In a method wherein a bulk superconductor is formed into the shape of a coil as disclosed in Japanese Patent Laid-Open no. Hei 5-175034, the working on the superconductor becomes considerably complicated and, if a ceramic superconductor is used, the working becomes very difficult and costly. Furthermore, deterioration of the material during the working is likely, thereby making it difficult to produce a superconductor having stable properties.
According to the foregoing conventional methods, even though bulk superconductors with good properties are available, it is difficult to use such bulk superconductors as magnets that produce great magnetic fields in various appliances and machines.
Japanese Patent Laid-Open No. Hei 6-168823 describes a method that applies pulse-like magnetic fields to a superconductor instead of a steady magnetic field. This method is very useful to magnetize a superconductor using a simple coil device.